In general, a high-performance portable power supply has been used as a main component of end-products essentially used in all portable information communication equipment, electronic apparatus, electric automobiles and so on. Next generation energy storage systems, which are recently developed, utilize electrochemical principles and are exemplified by a Li based secondary cell and an electrochemical capacitor.
An electrochemical capacitor is an energy storage device that stores and supplies electric energy using the capacitor behavior caused by the electrochemical reaction between an electrode and an electrolyte. In addition, the electrochemical capacitor is a new concept energy storage power source that can rapidly store and supply a large amount of energy as being excellent in terms of energy density and power density, compared to the conventional electrolytic capacitor and the secondary cell, and thus recently caught many people's attention.
The electrochemical capacitor is expected to be applied as a back-up power source of the electronic apparatus, a pulse power source of the portable mobile communication equipment, and a high-output power source of a hybrid electric automobile in many industrial fields owing to the characteristics of being capable of supplying a large amount of current within a short time
Among the electrochemical capacitors, the development of a super capacitor that has a larger energy density than that of the conventional capacitor comes into the spotlight.
An example of a representative super capacitor includes an electrical double layer capacitor (EDLC) using an electrical double layer formed between the electrode and the electrolyte, and a pseudo-capacitor that is generated from the faradaic reaction accompanied by the transportation of electric charges between the electrode and the electrolyte, such as the adsorption reaction of ions in the electrolyte on the surface of the electrode or the oxidation/reduction reaction of the electrode. The pseudo-capacitor is a super capacitor showing ultrahigh capacitance which has maximal capacitance 10-fold larger than that of the EDLC.
A metal oxide or conductive polymer is mainly used as an electrode material of the capacitor, and transition metal oxide materials come into the most significant spotlight as the electrode material of the super capacitor. In particular, ruthenium oxide exhibits very high specific capacitance, long operation time, high electrical conductivity, and excellent high-rate property in the aqueous electrolyte, and thus a research on the ruthenium oxide is the most actively in progress.
However, in case of using such an aqueous electrolyte, there is a disadvantage in that the operating voltage of the aqueous electrolyte is limited to 1V, and thus the energy density of the capacity is also limited. For this reason, the development of electrode materials such as vanadium oxide, manganese oxide or nickel oxide, cobalt oxide, and the like is vigorously carried out which can be used in an organic electrolyte whose operating voltage is at least 2.3V. However, these alternative electrode materials have not yet exhibited the electrochemical characteristics corresponding to those of ruthenium oxide so far.
In the meantime, as an attempt to improve the electrochemical characteristics of the metal oxide electrode, a global research is in progress on a carbon material-metal oxide composite electrode formed by mixing a metal oxide electrode material having a high specific capacitance and a carbon-based material having an excellent electrical conductivity
According to reports, a carbon material-metal oxide composite electrode could be prepared by a pasting technique. A carbon material/metal oxide material in the form of paste can be prepared by adding the carbon-based material when synthesizing a metal oxide and then mixing a conductive material and a binder. Alternatively, a synthesized metal oxide, a conductive material and a binder are mixed with the carbon material into a paste form, and then the paste material is applied on a current collector.
However, this pasting technique entails a problem in that a manufacturing process of carbon material-metal oxide composite electrode is a multi-staged process that is very complicated and requires a relatively long time, and in that the conductive material and the binder are indispensably used but these materials are not actually involved in the electrochemical reaction exhibiting the specific capacitance of the electrode.
Thus, the development of a material capable of improving energy density and power density and increasing the specific surface area of the electrode several hundred-fold among the conditions of the super capacitor will be the most critical factor for the preoccupancy of the technology of the next-generation capacitor.
Therefore, Japanese Patent Laid-Open Publication No. 1993-198461 discloses a porous metal electrode for a capacitor manufactured by forming an aluminum layer on a porous conductive gas diffusion layer using an electroplating method. In addition, Japanese Patent Application No. 1993-045947 discloses a porous structure electrode for a capacitor manufactured by performing electroplating on a roamed resin and then thermally treating the electroplated resin. Japanese Patent Application No. 2007-066819 discloses an electrode for a capacitor manufactured by sequentially laminating a nickel-plated layer and a chrome-plated layer on a porous non-woven fabric.
However, these Japanese patent documents suffer from a drawback in that since the plating is performed on the matrix formed with pores to manufacture the electrode for a capacitor, there is a limitation in the specific surface area of the electrode and the control of the specific surface area is impossible.
Accordingly, the present inventors have made extensive efforts to solve the problems occurring in the prior art, and as a result, have found that a nano-porous electrode for a super capacitor is manufactured using an electrodeposition method accompanied by hydrogen generation so that the specific surface area of the electrode can be controlled by a simple process and can also be increased, thereby completing the present invention.